1. Field of the Invention
The present invention relates to a fiber-type light conversion device.
2. Description of Background Information
An optical pickup device is known by which a high density writing and reading of information on and from an optical recording disk is enabled by transforming the wavelength of a laser beam emitted from a laser source to a half of the original wavelength by means of a light conversion device (a reference is directed to Japanese Patent Application Laid-Open No. 61-50122).
As the light conversion device for use in such an optical pickup device, there is a second harmonic generator (SHG) of an optical fiber type utilizing second-order non-linear optical effect. A phase matching of Cerenkov radiation system is adopted in the optical fiber-type SHG. With this system, it is possible to generate a second harmonic wave (SH wave) whose phase matching condition is attained almost automatically. The general conception of this fiber-type SHG is shown in FIGS. 1A and 1B.
In FIG. 1A, when a fundamental wave mode is propagated through the core of the fiber-type SHG with the effective refractive index of N (.omega.), the non-linear polarizing wave generating a SH wave is also propagated at the same phase velocity C/N (.omega.) (C is the speed of light). It is assumed that this non-linear polarizing wave produces the SH wave in a direction making an angle .theta. with respect to the direction of the wave guide at a point A, and generates the SH wave in the direction of .theta. as before at a point B, after the elapse of a unit time. If the SH wave generated at the point A propagates through the clad and reaches to a point C after the elapse of a unit time and the angle .theta. is such an angle that lines AC and BC are perpendicular to each other, then the plane of the SH wave which is generated from the non-linear polarized wave becomes equal to BC, and as a result, a coherent SH wave is generated.
The condition of the phase matching is, according to the figure, as follows: EQU N(.omega.)=N.sub.clad (2.omega.)cos.theta. (1)
where N.sub.clad (2.omega.) is the refractive index of the clad for the SH wave.
This in turn gives, EQU N(.omega.)&lt;N.sub.clad (2.omega.) (2)
This means that the SH wave is automatically generated in the direction .theta. where the phase matching condition is performed when at least the condition mentioned by the equation (2) is satisfied. Generally, with the refractive indexes of the clad and core with respect to the fundamental wave being n.sub.clad (.omega.) and n(.omega.), and with the air as the over-layer, the condition for the fundamental wave to propagate through the core as the mode is expressed as: EQU N.sub.clad (.omega.)&lt;N(.omega.)&lt;n(.omega.) (3)
Wavelength dispersion of the clad's refractive index will now be considered. Since n.sub.clad (.omega.)&lt;n.sub.clad (2.omega.), if the equation (2) is satisfied for all of the fundamental wave modes irrespectively of the diameter of the core so far as the following expression (4) is satisfied. EQU N.sub.clad (.omega.)&lt;N(.omega.)&lt;n.sub.clad (2.omega.) (4)
Moreover, there are fundamental modes satisfying the equation (2) in a certain range of the diameter of core even under the following condition. EQU N.sub.clad (.omega.)&lt;n.sub.clad (2.omega.)&lt;n(.omega.)
The SH wave generated in this way is propagated in a clad mode as illustrated in FIG. 1B in which total reflection occurs repeatedly at the boundary between the clad and air. Then, the SH wave is emerged in a hollow conical shape from the end of fiber in directions making an emergent angle .theta. relative to the fiber's direction. The equiphase wavefront of the SH wave emitted in this way is in a conical surface with an axis on the central axis of the fiber.
In order to utilize the SH wave as a light beam for writing and reading information on and from an optical recording disk as mentioned before, it is necessary to converge the light beam emerged from the fiber-type SHG onto the recording surface of the disk in the form of a small light spot. However, since the equiphase wavefront of the emergent light of SH wave is of the conical form, it is not possible to converge the emergent light to a degree near to a diffraction limit by only using a converging lens system constructed from conventional spherical lenses and/or aspherical lenses.
Therefore, as illustrated in FIG. 2, it is conceivable to dispose a conical prism 20 having a conical surface in the optical path of the emergent light beam 11 emitted from the fiber-type SHG 10 receiving primary light 13, so that the conical equiphase wavefront of the emergent light beam 11 of SH wave can be converted into secondary light 15 having a planer equiphase wavefront by collimating the SH wave, or making it parallel, by means of the function of the conical prism 20. With this arrangement, it becomes possible to converge the conical light beam up to the diffraction limit by means of a conventional converging lens.
However, in the case that the conical prism 20 is used in the optical pickup device, when the emergent angle of the SH wave emitted from the fiber-type SHG 10 is slipped from a desired angle, and/or the interior angle of vertex of the conical prism 20 is deviated from a desired angle, the conical prism 20 is not able to accurately collimate the received light beam of SH wave. As a result, an optical path of the secondary light beam of SH wave passing though the conical prism 20 does not become in parallel to the central axis of the optical system. In this case, the SH wave passing though the conical prism 20 is converged into a light ring spot by an objective lens or converging lens 21 as shown in FIG. 3. For example, if a SH wave emerging at an emergent angle .theta.' of 0.01.degree. after passing though the conical prism 20 and still having a conical equiphase wavefront, is converged by the objective lens 21 having a numerical aperture NA of 0.5 and a focal length f of 4.5 mm, then it makes a light ring spot having an inner diameter 0.16 .mu.m by means of the approximate equation 2f.tan.theta.' based on the geometrical optics. In view of the wave optics, if a conical prism 20 is used which has an interior angle of vertex deviated by only an angle of 0.05.degree. from the desired angle, then the whole optical system has a wavefront aberration of 0.07.lambda. r.m.s. As a result, it is difficult to utilize such a SH wave as a light beam for writing and reading information on and from an optical recording disk.
In this way, it is necessary to use a conical prism 20 having the interior angle of vertex with a great accuracy according to the emergent angle of the SH wave emitted from the fiber-type SHG 10 in order to collimate the SH wave. The production of the conical prism conical prism 20 is difficult because of the conical shape thereof as mentioned above, and the alignment between the SHG and the conical prism 20 is also difficult.